1. Field of the Invention
The present invention relates to a method of fabricating an ultra-small condenser microphone using a semiconductor process technology.
2. Description of the Related Art
An electret condenser microphone (ECM) is an acousto-electric transducer wherein an electret film having a semi-permanent electric polarity is formed by electretizing, and a DC (direct current) bias voltage is not needed to be applied to both electrodes of a condenser. An electret film is formed by electrically charging a dielectric film and fixing charges in the dielectric film so that a potential difference is generated between both electrodes by an electric field occurred by the fixed charges. Hereinafter, to fix charges in the dielectric film is referred to as ‘electretization’ and an amount of fixed charges is referred to as ‘an amount of deposited charges’.
FIG. 12 is a schematic cross-sectional view showing a principal part of a conventional electretization apparatus used for electretizing a dielectric film by injecting charges to form an electret film. In the apparatus shown in FIG. 12, a dielectric film is electretized by causing a corona discharge using a needle electrode. As shown in FIG. 12, a dielectric film 4 to be electretized is disposed on a ground electrode (a metal tray) 5. In this state, a high voltage power source 7 applies a DC voltage to a needle electrode 6 arranged opposite to the ground electrode 5, thereby causing the DC corona discharge between the needle electrodes 6 and the ground electrode 5. Electretization is performed by charging and fixing ions resulting from the DC corona discharge in the dielectric film 4 (for example, see Japanese Patent Laid-Open Publication No. 2007-294858).
In resent years, ultra-small condenser microphones have been fabricated by processing silicon substrates utilizing a micro-processing technology for semiconductor integrated circuits. Such ultra-small condenser microphones have received attention as micro-electro-mechanical system (MEMS) microphones (hereinafter referred to as MEMS microphones). MEMS microphones are incorporatedly formed on a silicon substrate using a semiconductor process technology, so that it is impossible that a dielectric film alone is taken out from the microphones and separately electretized. Therefore, the electretization apparatus shown in FIG. 12 can not be adopted.
The above mentioned Japanese Patent Laid-Open Publication No. 2007-294858 discloses that a dielectric film is electretized in a state that a MEMS microphone chip formed by micro-processing a silicon wafer is mounted on a substrate for packaging or in a state of an individual MEMS microphone chip which is separated by cutting a semiconductor substrate. In this technology, a dielectric film provided in a MEMS microphone chip is electretized by applying a corona discharge at least a time to a single or several MEMS microphone chips simultaneously by a needle electrode or a wire electrode.
FIG. 1 is a cross-sectional view showing a structure of a MEMS microphone chip fabricated by processing a silicon wafer using a semiconductor process technology. As shown in FIG. 1, a MEMS microphone chip 43 has a base 34 made of a silicon wafer (silicon diaphragm) having an opening in the center part. The opening in the base 34 is blocked by a vibration film 33. An inorganic dielectric film 32 which is an object to be electretized is formed on a surface of the vibration film 33 opposite the surface in contact with the base 34 located. A fixed electrode 31 supported by a spacer 37 is arranged to be opposed to the inorganic dielectric film 32. The fixed electrode 31 has a plurality of acoustic holes 35 (openings for transmitting an acoustic wave to the vibration film 33). An air gap 36 is provided between the inorganic dielectric film 32 and the fixed electrode 31. The air gap 36 is formed by etching and removing a sacrificial layer embedded that area in a fabricating process of the silicon wafer. In this structure, the vibration film 33 functions as one of electrodes of a condenser and the fixed electrode 31 functions as the other one of electrodes of the condenser. The silicon wafer supports only the outskirts of the vibration film 33 and the surface of the vibration film 33 is exposed from the opening of the silicon wafer.
FIG. 2 is a plan view showing the MEMS microphone chip 43 in FIG. 1. FIG. 1 is a cross-sectional view taken along A-A line in FIG. 2. As shown in FIG. 2, pads 40, 41 and 42 are formed on the surface having the inorganic dielectric film 32 in the base 34. The pad 40 is electrically connected to the fixed electrode 31 (not shown). The pad 41 is connected to the vibration film 33 by an interconnection passed through the spacer 37, and the pad 42 is electrically connected to the silicon wafer 34. The pads 40, 41 and 42 are utilized to make contact with probe pins in inspections and are utilized for wire bonding in assembling.